This invention relates to the deposition of materials, and, more particularly to the deposition of semiconductor layers for solar cells.
In the manufacture of multiple layered devices, the successive layers are stacked upon one another with each layer contributing a particular characteristic to the overall structure.
In the case of a typical solar cell, the structure is a layered laminate with an intermediate layer of intrinsic material sandwiched between respective outer layers of "n" type and "p" type material.
Each layer can be deposited from a suitable gaseous mixture in a reaction chamber. The reaction can take place in a variety of ways. For example, amorphous silicon can be formed from a gaseous mixture of lower and higher order silanes with diluents and additives, e.g. "dopants" that are selected in accordance with the characteristics desired in the layer that is to be produced.
In any case, it is necessary to form the layers in sequence. Thus the reaction chamber can be first used to produce an outer layer of the solar cell, i.e. either an "n" or "p" layer. The chamber is then evacuated and used to produce an intrinsic or "i" layer upon the previously formed "n" or "p" layer.
Unfortunately in the usual case, a certain amount of undesired doping of the i-layer takes place as a result of cross contamination. In particular, in the case of amorphous silicon solar cells, when diborane is used as a dopant gas in the formation of a p-layer, the diborane cannot be totally removed from the vacuum system and a substantial amount of boron is incorporated into the i-layer. The result is a surface concentration of boron that can extend into the i-layer for several hundred Angstroms. There is also an undesirable residual doping effect throughout the i-layer. The consequence is to reduce the response of the solar cell to certain portions of the excitation spectrum. Thus, the blue response of the cell can be significantly impaired with a resultant reduction in the efficiency of the cell in converting light energy to electricity.
In general the long term operation of the chamber leads to a build-up of deposits of semiconductor film, possibly including powder, that are undesirable. These deposits may increase the cross-contamination, and produce objectionable flaking that can cause pinholes in the deposited film.
Although the contamination can be reduced by extensive pumping and flushing, or by counterdoping, these procedures are costly, time consuming and not always reliable. In addition, counterdoping can have an adverse effect on the quality of the various layers in the resulting solar cell.
To eliminate the undesired deposits, it is necessary to subject the chamber to periodic cleaning. In the case of some chamber geometries, the cleaning operation becomes very difficult.
In addition the typical chamber is provided with only one substrate surface, either a cathode or anode, on which high quality device grade amorphous silicon can be deposited Such a system is asymmetric by virtue of the large difference in surface area between the cathode and the anode. The asymmetry makes it virtually impossible to realize device grade deposits simultaneously on both surfaces. Another common drawback of such deposition systems is the fact that the deposit is usually non-uniform in thickness across the substrate and is often substantially different in thickness at the periphery of the substrate. Such thickness variations are highly undesirable.
Accordingly, it is an object of the invention to facilitate the deposit of materials, particularly in successive layers. A related object is to facilitate the deposit of layers of amorphous silicon to form solar cells.
Another object of the invention is to achieve a high degree of silicon utilization and limit the cross-contamination between layers during their formation. A related object is to do so for layered structures of amorphous silicon. Another related object is to limit undesirable doping of intrinsic layers in multilayer structures, particularly for amorphous silicon solar cells.
Another related object of the invention is to curtail the extent to which a dopant gas used in the formation of a layer can affect an adjoining layer, particularly in the case of an "i" layer of amorphous solar cells. Still another related object is to overcome undesired transitional doping in layered structures particularly that which reduces the spectral response of solar cells.
Another object is to eliminate the need for cleaning the chamber. Still another object is to eliminate film nonuniformity and "pin holes" caused by the flaking of accumulated deposits.
A further object of the invention is to limit the need for pumping and flushing in order to reduce the effects of cross-contamination during the formation of layered structures.
A still further object of the invention is to avoid the need for counterdoping of layers to overcome the adverse effects caused by the inadvertent incorporation of adverse dopants.
Still another object of the invention is to realize a high throughput for a chamber of prescribed size, i.e. increase the amount of surface area that is subject to device quality deposit per unit of time. A related object is to realize device grade deposits simultaneously on both cathode and anode substrates.
Yet other aspects are to reduce outgassing from the chamber walls, to achieve a comparatively uniform temperature across all substrates, and to achieve a suitably uniform deposition of material on all substrates regarding both thickness and other properties.
Still other objects are to achieve a system which can be used for both chemical vapor deposition and glow discharge deposition; to achieve vertical deposition with a large percentage of active substrate, without cross contamination using symmetrical surfaces with heating that causes uniform deposition on electrodes.